Tag themes with properties

Tag themes with properties

* doc/emacs/custom.texi (Custom Themes): Document 'theme-choose-variant'.
* doc/lispref/customize.texi (Custom Themes): Document the new
optional argument to 'deftheme'.
(Autoload): Mention that 'deftheme' is not copied verbatim.
* etc/themes/adwaita-theme.el (adwaita): Add properties.
* etc/themes/deeper-blue-theme.el (deeper-blue): Add properties.
* etc/themes/dichromacy-theme.el (dichromacy): Add properties.
* etc/themes/light-blue-theme.el (light-blue): Add properties.
* etc/themes/manoj-dark-theme.el (manoj-dark): Add properties.
* etc/themes/misterioso-theme.el (misterioso): Add properties.
* etc/themes/tango-dark-theme.el (tango-dark): Add properties.
* etc/themes/tango-theme.el (tango): Add properties.
* etc/themes/tsdh-dark-theme.el (tsdh-dark): Add properties.
* etc/themes/tsdh-light-theme.el (tsdh-light): Add properties.
* etc/themes/wheatgrass-theme.el (wheatgrass): Add properties.
* etc/themes/whiteboard-theme.el (whiteboard): Add properties.
* etc/themes/wombat-theme.el (wombat): Add properties.
* etc/themes/modus-operandi-theme.el: Add properties.
* etc/themes/modus-vivendi-theme.el: Add properties.
* etc/themes/leuven-dark-theme.el (leuven-dark): Add properties.
* etc/themes/leuven-theme.el (leuven): Add properties.
* lisp/custom.el (deftheme): Allow for optional arguments to set the
property list.
(custom-declare-theme): Accept the same optional arguments as 'deftheme'.
(theme-list-variants): Add new function.
(theme-choose-variant): Add new command for switching between members
of a theme family.
(toggle-theme): Add an alias for 'theme-choose-variant'.
* lisp/emacs-lisp/loaddefs-gen.el (loaddefs-generate--make-autoload):
Handle 'defcustom's by extracting the properties.  (Bug#57639)